(1) Field of the Invention
This invention relates to a semiconductor device manufacturing method, wafer, and wafer manufacturing method and, more particularly, to a semiconductor device manufacturing method in which heat treatment is performed in the process of manufacturing semiconductor devices, a wafer on which such heat treatment is performed, and a method for manufacturing such a wafer.
(2) Description of the Related Art
To manufacture semiconductor devices, heat treatment, such as a spike anneal which needs a rapid increase and decrease in temperature, is currently performed on wafers. In this case, lamp annealers in which heat treatment is performed by irradiating a wafer with light emitted from a lamp (lamp light) are widely used.
Heat treatment is performed on a wafer with such a lamp annealer in the following way. First, a silicon (Si) wafer, for example, of predetermined size is placed on a ring-like substrate holder of predetermined size located in the chamber of the lamp annealer so that it will be supported by the edge portion. Then heat treatment is performed by irradiating a predetermined surface of the wafer placed in this way with lamp light.
With currently used lamp annealers, an area irradiated with lamp light is divided into a plurality of zones and each zone is irradiated with lamp light of proper intensity. Temperatures at a plurality of points on a wafer are monitored and are reflected in the intensity of lamp light with which each zone is irradiated. To make in-plane temperature distribution on the wafer as uniform as possible, a currently used lamp annealer irradiates each zone with lamp light of proper intensity, while adjusting the balance of the intensity of lamp light with which the plurality of zones are irradiated.
With lamp annealers, the temperature of a wafer is increased and decreased by lamp light. Conventionally, techniques for adjusting the reflection factor of a wafer irradiated with lamp light have been proposed in order to make in-plane temperature distribution on the wafer uniform. For example, the reflection factors of surfaces irradiated directly with lamp light differ among different wafers, so ultimate temperatures also differ among them. In order to solve this problem, a technique for making the reflection factor of a surface of each wafer irradiated with lamp light constant or for controlling the roughness, for example, of a surface of a wafer irradiated with lamp light in such a way that the reflection factor of the surface becomes lower with distance from the center is proposed (see Japanese Unexamined Patent Publication No. 9-246202). In addition, to prevent a slip line from appearing at the time of heating for a long time, a technique for working a wafer in such a way that a reflection factor in the central portion differs from a reflection factor in the edge portion is proposed (see Japanese Unexamined Patent Publication No. 60-732).
However, the following problems arise about the conventional method for performing heat treatment with a lamp annealer.
When semiconductor devices are manufactured, usually patterns for the semiconductor devices (which may not be complete) are formed on a wafer except the edge portion. If device patterns are formed in the edge portion by using a resist, the possibility that particles are produced in the edge portion in the process of manufacture gets greater. To prevent such particles from being produced, exposure is currently performed on the edge portion (area with a width from the edge of about 1 to 2 mm) of the wafer to remove the resist therein before predetermined device patterns are formed. This operation is repeated from the early stages to prevent the device patterns from being formed in the edge portion of the wafer. Accordingly, after many processes are performed, device patterns are formed on the wafer except the edge portion and an Si surface gets exposed in the edge portion.
The wafer where the device patterns are formed in this way is referred to as a device wafer. There are polycrystalline silicon used mainly as gate electrodes and silicon oxide used as isolation areas and the like in an area (device formed area) on this wafer where the device patterns are formed. As described above, the Si surface is in an exposed state in the edge portion outside the device formed area. The average reflection factor of the device formed area including polycrystalline silicon and silicon oxide is different from the reflection factor of the edge portion where the Si surface is in an exposed state.
It is assumed that the average reflection factor of the device formed area is lower than the reflection factor of the edge portion where the Si surface is in an exposed state. When heat treatment is performed on the device wafer with the intensity of lamp light in the lamp annealer uniform to increase and decrease the temperature of the wafer, the temperature of the edge portion is likely to be lower than that of the device formed area due to the difference in reflection factor. This may lead to a difference in final characteristic between a semiconductor device obtained from the central portion of the device wafer and a semiconductor device obtained from a portion near the edge portion of the device wafer because their thermal budgets are different from each other.
As described above, with the currently used lamp annealers each zone is irradiated with lamp light. Temperatures monitored at a plurality of points on a wafer are reflected in the intensity of lamp light with which each of zones from the center to the edge portion is irradiated. The currently used lamp annealers include lamps for directly irradiating the edge portion of a wafer and substrate holders with light. With the currently used lamp annealers, however, the temperature of substrate holders is not monitored to reflect it in the intensity of lamp light. Therefore, in the currently used lamp annealers, temperatures monitored in a device formed area near the edge portion on a wafer are reflected in the intensity of lamp light with which the edge portion and the substrate holders are irradiated. However, if the temperature of the edge portion the reflection factor of which is high and the temperature of which is difficult to raise is controlled on the basis of the temperature of the device formed area the reflection factor of which is low and the temperature of which is easy to raise, the amount of control is small and the temperature of the edge portion becomes relatively low. Accordingly, it is difficult to keep in-plane temperature distribution on the wafer uniform.
Moreover, in a lamp annealer, a substrate holder for supporting a device wafer by the edge portion is usually made from a material which is superior to the device wafer in heat resistance and its reflection factor and heat capacity are higher than those of the device wafer. Accordingly, even if each zone is irradiated with lamp light, the temperature of the edge portion of the device wafer is influenced by the temperature of the substrate holder. As a result, the temperature of the edge portion of the device wafer becomes lower than that of a device formed area and it is impossible to keep in-plane temperature distribution on the device wafer uniform.
In-plane temperature distribution on a wafer can be optimized by reflecting monitored temperatures in the intensity of lamp light and by adding an offset value to the intensity of lamp light. Conventionally, such an offset value has been calculated by using a bare wafer. That is to say, a bare wafer is treated in advance in a lamp annealer where a device wafer is to be treated. An optimum offset value to be added to the intensity of lamp light with which each zone is irradiated at the time of the device wafer being treated is set in advance on the basis of in-plane temperature distribution obtained at this time. However, the reflection factor of the bare wafer is the same as that of an exposed Si surface on the device wafer and is different from the average reflection factor of a device formed area. Therefore, if the offset value set on the basis of the bare wafer is used, the ultimate temperature of the device formed area at lamp anneal time will not be a proper value.
As described above, with the conventional method for performing heat treatment with a lamp annealer, it is difficult to make the temperatures of a device formed area and an edge portion having different reflection factors equal.